Recruitment of motor units in voluntary muscle contraction takes place in an orderly fashion. This process is dominated by differences in motor unit (alpha motoneuron) size and by spinal mechanisms regulating the motoneuron firing rate as a function of its excitatory state. Motor unit sizes, firing rates and recruitment thresholds have been measured precisely in a number of human muscle groups. This is accomplished by recording individual motor unit action potentials during a voluntary contraction and by extracting the unit contribution from the total muscular activity by means of an averaging technique. Unit size estimates are also derived from gradients in the firing rate behavior of recruited units, as a function of voluntary effort. Differences among motor units, in terms of twitch contraction time, fatigue resistance and the degree of motor unit synchronization, are measured as well. Combined with a functional representation of the muscle contractile response, these data complete an accurate model of the normal voluntary force production process. An automated technique has been developed to extract from the data, by computer, a reliable estimate of the motor unit size composition of several human skeletal muscles and muscle groups. Knowledge of the behavior of a relatively small number of motor units suffices for obtaining a clinically useful measure of the degree of normality of the motor unit composition. It is proposed in this supplement to: 1) study the relationship between motoneuron electrical properties and contractile properties of innervated muscle fibers for consistency in the course of long-term changes in the level of activation; and 2) complete a number of control experiments on the volume conduction of motor unit potentials in order to improve the accuracy of unit size measurement by multiple surface electrode mapping.